The present invention relates to an apparatus for manufacturing a spring unit used in a mattress or a box spring mattress.
A spring or assembly structure for use in a box spring mattress is well known. In this conventional spring structure, a number of main spring units are arranged on a rectangular base. Each main spring unit is formed of a wire and has a straight section and a pair of spring sections having upper ends connected to the two ends of the straight section. The straight sections of the main spring units are assembled in a lattice form. The lower ends of the spring sections are fixed to the base. The straight sections of the main spring units in a lattice form are reinforced by intermediate support spring units. As in the main spring units, each intermediate support spring unit has a straight section and a pair of spring sections having upper ends connected to the two ends of the straight section. The upper ends of each intermediate support spring unit are also connected to the straight section of a main spring unit, with the lower ends fixed to the base.
In the spring assembly having the above structure, the spring sections with the main and intermediate support spring units comprise torsion bar spring sections. However, a torsion bar spring section is subject to a small amount of deformation when placed under a compression load. In other words, although the spring sections are stiff enough to securely support the body of a user, they are easily subject to permanent distortion upon repeated application of the compression load. Therefore, the spring assembly loses elasticity after only a short period of use.
In view of the above problem, it has been proposed to replace the spring sections of the spring assembly with coil spring sections. Since coil springs collapse less easily than torsion bar springs, the spring units can maintain elasticity over a longer period of time.
An apparatus for manufacturing a spring unit having coil spring sections at two ends of a straight section from a single wire has been proposed in U.S. Ser. No. 626,435, filed June 29, 1984. Although this apparatus is suitable for mass production of spring units, it has a large number of steps for forming a single wire into a spring unit. Therefore, an apparatus which can manufacture a spring unit with simplified steps is desirable.